Do you have any tool windows set to autohide? There is a known issue with autohiding toolwindows in Visual Studio Beta 2. Here is a Product Feedback link:
http://lab.msdn.microsoft.com/ProductFeedback/viewFeedback.aspx?feedbackid=302e962d-5426-49d2-aa79-658b8d743a29
If this is the same problem it has been fixed in our latest builds.
Thanks,
Dylan Lingelbach
SDE
Visual Studio Core Editor Team